Try to avoid shooting the rear of a person unless you really have some particular idea in mind that would be impossible to convey without shooting from the front.
In this case, it seems more like you were too shy to shoot from the front and hence took her rear as IMO the shot from the rear here doesnt work because of uninteresting foreground or the seeming lack of motive. Try going round and shooting her front or side profile at least, perhaps a lower angle might be nice here too.
If you're a photographer on sports day, go all out to shoot people around the track as well. Being a large event, most people will gamely strike a pose for you or react spontaneously to the camera in a positive way. It's fine if they notice you. You'll get candids later when they become accustomed to having a photog. walking around.